Triplet-benzene-sensitized

Si —A, - Ti process. This does not rule out the possibility of the Ti- Sq process followed by the molecular elimination process (II) as proposed earlier (162) when Tp is produced at low energies by triplet benzene sensitization. 

A similar biradical mechanism was postulated in an earlier study of the triplet-benzene-sensitized decomposition of 4-pentenal (136). This mechanism requires an efficient intramolecular... 

The formation of benzvalene from the irradiation of benzene was first reported in 1967,8 but the process continues to receive further study. The quantum yield (0.18) for formation of this valence-bond isomer from 254 nm radiation of oxygen-free solutions of benzene in hexane is reported to be independent of temperature within the range 9—50 °C. The benzvalene is estimated as 5,6-di-bromobicyclo[2,l,l]hex-2-ene and the results are discussed in terms of a kinetic model which incorporates the known step of triplet-benzene-sensitized disappearance of the benzvalene. Light-induced transposition of ring substituents in... 

The butene method is the most straightforward and it has been the more thoroughly explored of the two methods. In essence, triplet benzene sensitizes cis- or rrc H.9-butene-2, and in the subsequent relaxation of triplet butene, isomerization occurs. The quantum yield of this isomerization can then be traced back to the quantum yield of triplet benzene formation. 

Carless, J. Metcalfe, and E.K.C. Lee (1972), Photolytic and triplet benzene-sensitized... 

Direct flash excitation (>400 nm) or the triplet-acetone-sensitization of nitrosamide X in degassed water or benzene solutions gives the amidyl radical transient exhibiting lax 335-350 nm. This transient is not observed with undegassed solution of X, indicating that oxygen has intercepted the precursor of the amidyl radical at least, with the diffusion controlled rate... 

In benzene solution, quantum yields for formation of olefin triplets from 2537 A irradiation are only 0.20.235 There is some doubt whether triplet benzene is the actual energy donor in solution, since the quantum yield for sensitized phosphorescence of biacetyl depends on a greater than first order of benzene concentration.238 Dubois has suggested that an excited dimer is responsible for energy transfer.236... 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

It should be mentioned in passing that both mercury and benzene sensitized Norrish Type II reactions have been shown to occur. Under the proper conditions the triplet states of the substrate molecules would be preferentially excited. This is further proof that Type II reactions may, with certain compounds, occur through the triplet state. 

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

Re-examination of the benzene-sensitized isomerization of alkenes revealed that the photostationary EjZ ratio critically depends on the structure of alkenes and the triplet energy of sensitizers [69]. For instance, thermal E-Z equilibrations of 2-octene 19c and 3,4-dimethyl-2-pentene 20 give the thermodynamic EjZ ratios of 3.3 and 3.5, respectively, while the photostationary E Z ratios upon benzene sensitization are 1.1 and 1.6, respectively (Schs. 9 and 10). For practical runs, the use of /7-xylene or phenol, rather than benzene, in ether is recommended, as the yellowing of the irradiated solution, which retards the photoreaction, is substantially decreased. 

Triplet-triplet absorption by benzene has never been detected and evidence for the presence of triplet benzene in fluids has been obtained only recently using energy transfer measurements. Benzene sensitizes both the fluorescence and the phosphorescence of biacetyl. The concentration dependence of this sensitized luminescence has been shown to be consistent with the presence of triplet benzene in the gas phase43 and in cyclohexane solution,80 though another study using hexane as solvent did not yield any evidence for the presence of triplet benzene,27 presumably because of impurities in the hexane. 

Kistiakowsky and Parmeter studied the benzene-sensitized isomerization of CTS-butene-2 at one pressure, viz., 0.047 mm. They found isomerization even at this low pressure, thus indicating that triplet-state benzene molecules are present in CeHe irradiated at this low pressure. They did not study the effect of pressure by the Cundall technique. They did, however, study the relative emission from benzene as a function of pressure at low pressures. By making the assumption supported within experimental error by the data of Ishikawa and Noyes that all absorbing molecules either emit or cross over to the triplet state, Kistiakowsky and Parmeter conclude that the rate of crossover is independent of collisions even at these low pressures. 

In liquid phase photosensitization experiment it was observed that triplet photo-sensitizers (benzene, benzophenone), in spite of efficient energy transfer to azo-... 

Since (E)-cyclooctene (20E) is the smallest member of the (E)-cycloalkenes isolable under ambient conditions, its photochemical behavior has been studied intensively as a prototype or model system of the cycloalkene family selected results are summarized in Table 16.1. When (Z)-cyclooctene (20Z) is irradiated with an equimolar amount of benzene or naphthalene, 1 1 sensitizer-substrate adducts rather than the (E)-isomer are produced.Upon xylene sensitization, however, the adduct formation is retarded and the geometrical isomerization dominates to afford 20E in an acceptable isolated yield. The triplet-sensitized photoisomerization of 20Z has been studied in some detail and has established that the EtZ ratio depends on the alkene concentration. The concentration dependence observed is attributed to the involvement of the excited singlet state, which tends to give arene-cyclooctene adducts such as 49 and 50. Temperature and sohrent-viscosity effects upon triplet-sensitized photoisomerization of cyclooctene have been studied in deta0. In the alkyl-benzene-sensitized isomerization of 20Z in pentane, the photostationary-state E/Z ratio is substantially affected by temperature, being doubled when the temperature is lowered from 20 to —78°C. This result is in contrast with the photoisomerization behavior of acyclic 2-octene. 

Benzene-sensitized photoisomerization of (Z)-cyclododecene (34Z) affords 34E in a 39 61 E/Z ratio at the photostationary state. Other triplet sensitizers such as benzophenone, acetophenone, and acetone, as well as copper salts, can also be used. Sensitized photoisomerization of 1,5,9-cyclododecatriene has also been studied. Direct irradiation of (E)-3-methylenecyclodecene (55E) affords 55Z, which further pho-toisomerizes to yield 56, 57, and 58. Triphenylene sensitization of 55E affords a photostationary-state mixture of 55E and 55Z in a ratio of 94 6. Doubly-bridged ethylene derivatives 59Z (n = 8, 10), i.e., (Z)-bicyclo[8.8.0]octadec-l(10)-ene and (Z)-bicyclo[10.8.0]eicos-l(12)-ene, isomerize to the (E)-iso-mers upon xylene-sensitized or direct irradiation. An E/Z ratio of 0.42 is given upon sensitization of the latter, while much higher E/Z ratios of 9 and 2 are attained upon direct irradiation at 254 nm of (Z)-bicyclooctadecene and (Z)-bicycloeicosene, respectively. ... 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Similarly quantitative yields of (141) are obtained from (140) in benzene solution by triplet sensitization using acetophenone. 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

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